Neuronotrophic factors (NTFs) are a specialized group of proteins which function to promote the survival, growth, maintenance, and functional capabilities of selected populations of neurons. Recent studies have demonstrated that neuronal death occurs in the nervous systems of vertebrates during certain periods of growth and development. However, the addition of soluble neuronal trophic factors from associated target tissues serves to mitigate this phenomenon of neuronal death. The following citations discuss neuronal trophic factors and their disclosures are hereby incorporated by reference: Chau, R. M. W., et al., Neuronotrophic Factor, 6 Chin. J Neuroanatomy 129 (1990); Kuno, M., Target Dependence of Motoneuronal Survival: The Current Status, 9 Neurosci. Res. 155 (1990); Bard, Y. A., Trophic Factors and Neuronal Survival, 2 Neuron 1525 (1989); Oppenheim, R. W., The Neurotrophic Theory and Naturally Occurring Motoneuron Death, 12 TINS 252 (1989); Bard, Y. A., What, If Anything, is a Neurotrophic Factor?, 11 TINS 343 (1988); and Thoenen, H., and Edgar, D., Neurotrophic Factors, 229 Science 238 (1985).
In the vertebrate neuromuscular system, the survival of embryonic motoneurons have been found to be dependent upon specific trophic substances derived from the associated developing skeletal muscles. Skeletal muscles have been shown, by both in vivo and in vitro studies, to produce substances which are capable of enhancing the survival and development of motoneurons by preventing the embryonic motoneurons from degeneration and subsequent, natural cellular death. See O'Brian, R. J. and Fischbach, G. D., Isolation of Embryonic Chick Motoneurons and Their Survival In Vitro, 6 J. Neurosci. 3265 (1986); Hollyday, M. and Hamburger, V., Reduction of the Naturally Occurring Motor Neuron Loss by Enlargement of the Periphery, 170 J. Comp. Neurol. 311 (1976), whose disclosures are incorporated herein by reference. Similarly, several investigators have reported that chick and rat skeletal muscles possess certain trophic factors which can prevent the natural cellular death of embryonic motoneurons both in vivo and in vitro. See McManaman, J. L., et al., Purification of a Skeletal Muscle Polypeptide Which Stimulates Choline Acetyltransferase Activity in Cultured Spinal Cord Neurons, 263 J. Biol. Chem. 5890 (1988); Oppenheim, R. W., et al., Reduction of Naturally Occurring Motoneuron Death In Vitro by a Target Derived Neurotrophic Factor, 240 Science 919 (1988); and Smith, R. G., et al., Selective Effects of Skeletal Muscle Extract Fractions on Motoneurons Development In Vivo, 6 J. Neurosci. 439 (1986), whose disclosures are incorporated herein by reference.
In addition, a polypeptide has been isolated from rat skeletal muscle which has been found to selectively enhance the survival of embryonic chick motoneurons in vivo, as well the activity of choline acetyltransferase in these motoneurons. This polypeptide has been named Choline Acetyltransferase Development Factor (CDF) and its biological function has been demonstrated to be different from other trophic factors such as Nerve Growth Factor (NGF), Ciliary Ganglion Neurotrophic Factor (CNTF), Brain-Derived Neurotrophic Factor (BDNF), and Retinal Ganglion Neurotrophic Factor (RGNTF). See Levi-Montalcini, R., “Developmental Neurobiology and the Natural History of Nerve Growth Factor,” 5 Ann. Rev. Neurosci. 341 (1982); Varon, S., et al., Growth Factors. In: Advances in Neurology, Vol. 47: Functional Recovery in Neurological Disease, Waxman, S. G. (ed.), Raven Press, New York, pp. 493–521 (1988); Barde, Y. A., Trophic Factors and Neuronal Survival, 2 Neuron 1525 (1989); Chau, R. M. W., et al., The Effect of a 30 kD Protein from Tectal Extract of Rat on Cultured Retinal Neurons, 34 Science in China, Series B, 908 (1991), whose disclosures are incorporated herein by reference.
The isolation and characterization of two motoneuronotrophic factors from rat muscle tissue having apparent molecular weights of 35 kD and 22 kD were reported by Chau et al. See Chau, R. M. W., et al., Muscle Neuronotrophic Factors Specific for Anterior Horn Motoneurons of Rat Spinal Cord. In: Recent Advances in Cellular and Molecular Biology, Vol. 5, Peeters Press, Leuven, Belgium, pp. 89–94 (1992), the disclosure of which is hereby incorporated by reference. The 35 kD protein has been defined by Dr. Chau as motoneuronotrophic factor 1 (MNTF1) and the apparent 22 kD protein as motoneuronotrophic factor 2 (MNTF2). These two trophic factors have been demonstrated in vitro to support the growth and/or regeneration of both isolated anterior horn motoneurons and spinal explants of rat lumber spinal cord.
Subsequently, in 1993, Chau et al reported immunological screening of lambda gt11 clones from a human retinoblastoma cDNA library using a monoclonal antibody to MNTF1 as an immunoprobe. Immunoblots of extracts from an immunopositive clone stained an MNTF1 protein having an apparent molecular weight of 55 kD See Chau, R. M. W., et al., Cloning of Genes for Muscle-Derived Motoneuronotrophic Factor 1 (MNTF1) and Its Receptor by Monoclonal Antibody Probes, (abstract) 19 Soc. for Neurosci. part 1, 252 (1993), the disclosure of which is hereby incorporated by reference. An extract containing the cloned human MNTF1 was shown to have biological activity similar to that of the “native” MNTF1 protein in that it supported the in vitro growth of rat anterior horn motoneurons.
More recently, U.S. Pat. No. 6,309,877 disclosed a family of neuronotrophic factors which possess the ability to exert a trophic effect on motoneurons. The motoneurotrophic factors were isolated, nucleic acid sequences encoding these factors were cloned and expressed, and both the nucleic acid and polypeptide sequences were provided. In particular, recombinant proteins MNTF1–F3 and MNTF1–F6 encoded by 1443 and 972 base pair inserts, respectively, were expressed as either fusion proteins or purified fragments. The isolated factors and the expressed, recombinant factors, were capable of inducing the continued viability and neurite outgrowth of motoneurons. Therefore, these factors have been classified as “motoneuronotrophic factors” or “MNTFs.”
The MNTF1–F6 clones reported in U.S. Pat. No. 6,309,877 encode a 33 amino acid fragment of MNTF1. Recombinant protein containing this sequence reacted with monoclonal antibody to MNTF1, maintained motoneuron viability, increased neurite outgrowth, reduced cell death/apoptosis and supported the growth and “spreading” of motoneurons into giant, active neurons with extended growth cone-containing axons. Consequently, the following studies were conducted to determine if a peptide comprising a “minimal” active site can be synthesized, which still retains the biological activity of this MNTF1 fragment.